The instant invention relates to a device for optically indicating the pressure of a medium, in particular, the air pressure in a vehicle tire.
A device of this type is known from DE 40 07 388 C2. This known device has an optically transparent element which consists of a light-screening element and a balloon connected to same, said balloon being filled with a light-absorbing liquid. The optically transparent element interacts with a piston capable of being moved towards and away from the optically transparent element. The piston is coated with a color layer on its side facing towards the optically transparent element. The movement of the piston towards the optically transparent element is caused by the pressure of a medium pressing the piston in opposition to the force of a spring. The movement of the piston away from the optically transparent element is caused by the force of the spring when the pressure drops.
If no pressure or only little pressure is present and acting upon the piston so that no pressure is exerted by the piston upon the balloon of the optically transparent element, the entire flat light-screening element is filled evenly with the light-absorbing liquid in the balloon. In this state, the light reflected by the color layer on the piston is absorbed by the light-absorbing liquid. The light falling on the color layer of the piston is therefore screened by the light-absorbing liquid in the balloon so that the color layer is not visible from the outside.
If the piston is moved by the pressure of the medium towards the optically transparent element until the piston surface coated with the color layer is pressed against the balloon containing the light-absorbing liquid, the liquid recedes into the border zone of the balloon. The sides of the balloon facing each other come into contact with each other so that light is no longer absorbed by the light-absorbing liquid. The light reflected from the color layer of the piston traverses the optically transparent element so that the color layer of the piston becomes visible from the outside.
This known device has the disadvantage of being relatively expensive to make and also delicate. For example, leaks may occur in the balloon containing the light-absorbing liquid so that the liquid is lost and the device thereby becomes ineffective in indicating the pressure of a medium.
It is the object of the instant invention to provide a device for the optical indication of the pressure exerted by a medium which device is simple in construction as well as inexpensive and reliable in operation.